How do I go about changing my employer on my NAFTA work permit? Is it as simple as getting another offer letter and taking it to a border crossing?

A new work permit would be required to switch employers. This can be done wither at a visa office or at a port of entry (based on your citizenship in this case) with the necessary supporting documentation and fees.

I am American. My NAFTA permit doesn't expire until Nov 07. What I am trying to do is change employers well before the expiry. If I get an offer letter can I simply take it to the port of entry and change it over?